fix(qwen): correct context lengths for qwen3-coder models and send max_tokens to portal

Based on PR #7285 by @kshitijk4poor.

Two bugs affecting Qwen OAuth users:

1. Wrong context window — qwen3-coder-plus showed 128K instead of 1M.
   Added specific entries before the generic qwen catch-all:
   - qwen3-coder-plus: 1,000,000 (corrected from PR's 1,048,576 per
     official Alibaba Cloud docs and OpenRouter)
   - qwen3-coder: 262,144

2. Random stopping — max_tokens was suppressed for Qwen Portal, so the
   server applied its own low default. Reasoning models exhaust that on
   thinking tokens. Now: honor explicit max_tokens, default to 65536
   when unset.

Co-authored-by: kshitijk4poor <82637225+kshitijk4poor@users.noreply.github.com>

agent/model_metadata.py

@@ -113,7 +113,10 @@ DEFAULT_CONTEXT_LENGTHS = {
     "deepseek": 128000,
     # Meta
     "llama": 131072,
-    # Qwen
+    # Qwen — specific model families before the catch-all.
+    # Official docs: https://help.aliyun.com/zh/model-studio/developer-reference/
+    "qwen3-coder-plus": 1000000,  # 1M context
+    "qwen3-coder": 262144,        # 256K context
     "qwen": 131072,
     # MiniMax — official docs: 204,800 context for all models
     # https://platform.minimax.io/docs/api-reference/text-anthropic-api